RNA interference is rapidly becoming a powerful toot for genetic analyses in mammalian systems. A potential drawback to transient small inhibitory RNA silencing is the short duration of downregulation it confers, usually only 24-72 h. Viral-based vector systems for the long-term delivery of RNA hairpins have been developed, yet they require expertise in viral production and transduction. Here we describe a simple plasmid-based system for the generation of long-term gene knockdown utilizing RNA interference combined with the gene delivery capabilities of the mammalian Tcl-like transposon Sleeping Beauty. Designated Maleficent, this system is shown to downregulate exogenous expression of GFP in a constitutively positive cell line. In addition, targeting of the enclogenously expressed lamin A gene results in long-term silencing with significant reduction in protein levels (>95%). Maleficent therefore provides a relatively easy, efficient, and stable means of delivering RNAi hairpins to generate long-term gene-specific knockdown cell lines. (C) 2004 Elsevier Inc. All rights reserved.